"Freud's Last Session," the hit off-Broadway play now in commercial residence at the Mercury Theatre, replete with the original New York cast, imagines that a dying and exiled Sigmund Freud invites a young man to his study in London, on the very eve of World War II. The man is C.S. Lewis, one of the few Christian writers and allegorists with a perennial following among agnostics and non-believers. "I wanted to learn how a man of your intellect could suddenly abandon truth and embrace an insidious lie," the great man of science says to his nervous guest, once he arrives from Oxford. But Lewis gives as good as he gets. "There is a God," he argues, in numerous different ways, "and a man does not have to be an imbecile to believe in him."

So goes the argument of the night.

To a large extent, "Freud's Last Session," which was written by Mark St. Germain and suggested by a book called "The Question of God" by Armand M. Nicholai, Jr. is a scrupulously balanced play specifically designed to allow two of the greatest minds of the 20th century—one who saw all religious fantasies as flowing from hidden psychological need or desire, and the other a romantic man determined to make the intellectual case for faith—to go at each other for 90 minutes or so, pondering the really big and juicy questions about life in such a way that we ponder them in their wake. Even as merely such, it has great appeal for those who weary of intellectually trivial drama.

But despite its careful parallelisms and savvy set-ups, St. Germain's play still manages to be remarkably credible and rich in its portrait of these characters. We're used to seeing Lewis as an older, comfortable, tweedy and accomplished fellow—the dominant image in the play and movie "Shadowlands," for example—but in this instance, he's young and insecure. As played by the terrific Mark H. Dold, this Lewis is emotional, a tad smug, quick to anger and even faster to betray his insecurity. His conversion to Christianity is fresh in his mind: he argues it with passion, suggesting, for example, that if the Apostles were all about building a legend they would not have allowed so much of Jesus' life to remain a mystery, and that if Jesus were a deluded madman when he claimed to be the son of God, he would not have otherwise have had such a formidable grasp on reason.

But the patient at Freud's last session (make that patients, if we include the way Freud also has himself on his own couch) is not the Lewis we think we know, which makes this play all the more interesting. Dold also shows us a man who's not allowed the mystery of faith to roll around long in his head and has not yet figured out the little matters of unjust suffering and death. God's apparent lack of power or, maybe, indifference in the face of human pain is, he says, with some agony, "the hardest question of all."

Which is where, at the savvy moment of this play's setting, Freud finds himself. The famed analyst is dying of inoperable throat cancer, which is enough to inject tension and high stakes into the dramatic proceedings (this is no mere meeting) and, of course, to shows us the hardest moment in life for a non-believer: the point of our shuffling off this earthly coil. Martin Rayner snags all that very nicely, coming up with a portrait of a man clinging on to his lifetime of confident intellectual achievement even while staring into a new and intimidating abyss. It's a more comedic performance than the one offered by his partner and it sometimes pushes dangerously close to caricature. But it never steps over the line; Rayner knows when to pull back from the gags such a play needs, and the show's director, Tyler Marchant, doesn't stint from showing us Freud's suffering, even among all the droll bon mots and the familiar deep Austrian accent.

The other clever aspect of a well-paced show (a show you don't really want to end) is its setting on the very day that Britain entered the war, both an explanation of why Freud had to get out of Austria, and a reminder of one of central tenants of Freud's arguments: "Man's physical self evolves but not man's character." That's one of many things about which this modestly scaled but wholly engaging play will leave you thinking.